Impregnated drill bits and methods for making the same

ABSTRACT

An impregnated cutting structure that includes a plurality of first encapsulated particles, each first encapsulated particle comprising a first abrasive particle encapsulated by a first matrix material shell; and a plurality of second encapsulated particles, the second encapsulated particles comprising a second abrasive particle encapsulated by a second matrix material shell, wherein the first encapsulated particles and the second encapsulated particles have at least one property difference is disclosed.

BACKGROUND OF INVENTION

1. Field of the Invention

Embodiments disclosed herein relate generally to drill bits, and moreparticularly to drill bits having impregnated cutting surfaces and themethods for the manufacture of such drill bits.

2. Background Art

An earth-boring drill bit is typically mounted on the lower end of adrill string and is rotated by rotating the drill string at the surfaceor by actuation of downhole motors or turbines, or by both methods. Whenweight is applied to the drill string, the rotating drill bit engagesthe earth formation and proceeds to form a borehole along apredetermined path toward a target zone.

Different types of bits work more efficiently against differentformation hardnesses. For example, bits containing inserts that aredesigned to shear the formation frequently drill formations that rangefrom soft to medium hard. These inserts often have polycrystallinediamond compacts (PDC's) as their cutting faces.

Roller cone bits are efficient and effective for drilling throughformation materials that are of medium to hard hardness. The mechanismfor drilling with a roller cone bit is primarily a crushing and gougingaction, in which the inserts of the rotating cones are impacted againstthe formation material. This action compresses the material beyond itscompressive strength and allows the bit to cut through the formation.

For still harder materials, the mechanism for drilling changes fromshearing to abrasion. For abrasive drilling, bits having fixed, abrasiveelements are preferred. While bits having abrasive polycrystallinediamond cutting elements are known to be effective in some formations,they have been found to be less effective for hard, very abrasiveformations such as sandstone. For these hard formations, cuttingstructures that comprise particulate diamond, or diamond grit,impregnated in a supporting matrix are effective. In the discussion thatfollows, components of this type are referred to as “diamondimpregnated.”

Diamond impregnated drill bits are commonly used for boring holes invery hard or abrasive rock formations. The cutting face of such bitscontains natural or synthetic diamonds distributed within a supportingmaterial to form an abrasive layer. During operation of the drill bit,diamonds within the abrasive layer are gradually exposed as thesupporting material is worn away. The continuous exposure of newdiamonds by wear of the supporting material on the cutting face is thefundamental functional principle for impregnated drill bits.

The construction of the abrasive layer is of critical importance to theperformance of diamond impregnated drill bits. The abrasive layertypically contains diamonds and/or other super-hard materialsdistributed within a suitable supporting material. The supportingmaterial must have specifically controlled physical and mechanicalproperties in order to expose diamonds at the proper rate.

Metal-matrix composites are commonly used for the supporting materialbecause the specific properties can be controlled by modifying theprocessing or components. The metal-matrix usually combines a hardparticulate phase with a ductile metallic phase. The hard phase oftenconsists of tungsten carbide and other refractory or ceramic compounds.Copper or other nonferrous alloys are typically used for the metallicbinder phase. Common powder metallurgical methods, such as hot-pressing,sintering, and infiltration are used to form the components of thesupporting material into a metal-matrix composite. Specific changes inthe quantities of the components and the subsequent processing allowcontrol of the hardness, toughness, erosion and abrasion resistance, andother properties of the matrix.

Proper movement of fluid used to remove the rock cuttings and cool theexposed diamonds is important for the proper function and performance ofdiamond impregnated bits. The cutting face of a diamond impregnated bittypically includes an arrangement of recessed fluid paths intended topromote uniform flow from a central plenum to the periphery of the bit.The fluid paths usually divide the abrasive layer into distinct raisedribs with diamonds exposed on the tops of the ribs. The fluid providescooling for the exposed diamonds and forms a slurry with the rockcuttings. The slurry must travel across the top of the rib beforereentering the fluid paths, which contributes to wear of the supportingmaterial.

An example of a prior art diamond impregnated drill bit is shown inFIG. 1. The impregnated bit 10 includes a bit body 12 and a plurality ofribs 14 that are formed in the bit body 12. The ribs 14 are separated bychannels 16 that enable drilling fluid to flow between and both cleanand cool the ribs 14. The ribs 14 are typically arranged in groups 20where a gap 18 between groups 20 is typically formed by removing oromitting at least a portion of a rib 14. The gaps 18, which may bereferred to as “fluid courses,” are positioned to provide additionalflow channels for drilling fluid and to provide a passage for formationcuttings to travel past the drill bit 10 toward the surface of awellbore (not shown).

Impregnated bits are typically made from a solid body of matrix materialformed by any one of a number of powder metallurgy processes known inthe art. During the powder metallurgy process, abrasive particles and amatrix powder are infiltrated with a molten binder material. Uponcooling, the bit body includes the binder material, matrix material, andthe abrasive particles suspended both near and on the surface of thedrill bit. The abrasive particles typically include small particles ofnatural or synthetic diamond. Synthetic diamond used in diamondimpregnated drill bits is typically in the form of single crystals.However, thermally stable polycrystalline diamond (TSP) particles mayalso be used.

In one impregnated bit forming process, the shank of the bit issupported in its proper position in the mold cavity along with any othernecessary formers, e.g. those used to form holes to receive fluidnozzles. The remainder of the cavity is filled with a charge of tungstencarbide powder. Finally, a binder, and more specifically an infiltrant,typically a nickel brass copper based alloy, is placed on top of thecharge of powder. The mold is then heated sufficiently to melt theinfiltrant and held at an elevated temperature for a sufficient periodto allow it to flow into and bind the powder matrix or matrix andsegments. For example, the bit body may be held at an elevatedtemperature (>1800° F.) for a period on the order of 0.75 to 2.5 hours,depending on the size of the bit body, during the infiltration process.

By this process, a monolithic bit body that incorporates the desiredcomponents is formed. One method for forming such a bit structure isdisclosed in U.S. Pat. No. 6,394,202 (the '202 patent), which isassigned to the assignee of the present invention and is herebyincorporated by reference.

Referring now to FIG. 2, a drill bit 22 in accordance with the '202patent comprises a shank 24 and a crown 26. Shank 24 is typically formedof steel and includes a threaded pin 28 for attachment to a drillstring. Crown 26 has a cutting face 29 and outer side surface 30.According to one embodiment, crown 26 is formed by infiltrating a massof tungsten-carbide powder impregnated with synthetic or naturaldiamond, as described above.

Crown 26 may include various surface features, such as raised ridges 32.Preferably, formers are included during the manufacturing process sothat the infiltrated, diamond-impregnated crown includes a plurality ofholes or sockets 34 that are sized and shaped to receive a correspondingplurality of diamond-impregnated inserts 36. Once crown 26 is formed,inserts 36 are mounted in the sockets 34 and affixed by any suitablemethod, such as brazing, adhesive, mechanical means such as interferencefit, or the like. As shown in FIG. 2, the sockets can each besubstantially perpendicular to the surface of the crown. Alternatively,and as shown in FIG. 2, holes 34 can be inclined with respect to thesurface of the crown 26. In this embodiment, the sockets are inclinedsuch that inserts 36 are oriented substantially in the direction ofrotation of the bit, so as to enhance cutting.

As a result of the manufacturing technique of the '202 patent, eachdiamond-impregnated insert is subjected to a total thermal exposure thatis significantly reduced as compared to previously known techniques formanufacturing infiltrated diamond-impregnated bits. For example,diamonds imbedded according to methods disclosed in the '202 patent havea total thermal exposure of less than 40 minutes, and more typicallyless than 20 minutes (and more generally about 5 minutes), above 1500°F. This limited thermal exposure is due to the shortened hot pressingperiod and the use of the brazing process.

The total thermal exposure of methods disclosed in the '202 patentcompares very favorably with the total thermal exposure of at leastabout 45 minutes, and more typically about 60-120 minutes, attemperatures above 1500° F., that occurs in conventional manufacturingof furnace-infiltrated, diamond-impregnated bits. If diamond-impregnatedinserts are affixed to the bit body by adhesive or by mechanical meanssuch as interference fit, the total thermal exposure of the diamonds iseven less.

With respect to the diamond material to be incorporated (either as aninsert, or on the bit, or both), diamond granules are formed by mixingdiamonds with matrix power and binder into a paste. The paste is thenextruded into short “sausages” that are rolled and dried into irregulargranules. The process for making diamond-impregnated matrix for bitbodies involves hand mixing of matrix powder with diamonds and a binderto make a paste. The paste is then packed into the desired areas of amold. The resultant irregular diamond distribution has clusters with toomany diamonds, while other areas are void of diamonds. The diamondclusters lack sufficient matrix material around them for good diamondretention. The areas void or low in diamond concentration have poor wearproperties. Accordingly, the bit or insert may fail prematurely, due touneven wear. As the motors or turbines powering the bit improve (highersustained RPM), and as the drilling conditions become more demanding,the durability of diamond-impregnated bits needs to improve. However,generally, as durability of a bit increases (with a harder matrix),diamond exposure (and thus ROP) generally decreases, and vice versa.Accordingly, there exists a continuing need for improvements in diamondimpregnated cutting structures to improve wear properties, rate ofpenetration, and diamond distribution.

SUMMARY OF INVENTION

In one aspect, embodiments disclosed herein relate to an impregnatedcutting structure that includes a plurality of first encapsulatedparticles, each first encapsulated particle comprising a first abrasiveparticle encapsulated by a first matrix material shell; and a pluralityof second encapsulated particles, the second encapsulated particlescomprising a second abrasive particle encapsulated by a second matrixmaterial shell, wherein the first encapsulated particles and the secondencapsulated particles have at least one property difference.

In another aspect, embodiments disclosed herein relate to a drill bitthat includes a bit body; and a plurality of ribs formed in the bitbody, wherein at least one rib comprises: a plurality of firstencapsulated particles, each first encapsulated particle comprising afirst abrasive particle encapsulated by a first matrix material shell; aplurality of second encapsulated particles, each second encapsulatedparticle comprising a second abrasive particle encapsulated by a secondmatrix material shell, wherein the first encapsulated particles and thesecond encapsulated particles comprise at least one property differencetherebetween.

In another aspect, embodiments disclosed herein relate to drill bit thatincludes a bit body; and a plurality of ribs formed in the bit body,wherein a portion of at least one rib has a height to width ratio ofgreater than about 1.75 with a minimum diamond concentration of 100 andcomprises: a plurality of first encapsulated particles, each firstencapsulated particle comprising a first abrasive particle encapsulatedby a first matrix material shell.

In yet another aspect, embodiments disclosed herein relate to a methodof forming an impregnated cutting structure that includes loading aplurality of first encapsulated particles and a plurality of secondencapsulated particles into a mold cavity, each first encapsulatedparticle comprising a first abrasive particle encapsulated by a firstmatrix material shell and each second encapsulated particle comprising asecond abrasive particle encapsulated by a second matrix material shell,wherein the first encapsulated particles and the second encapsulatedparticles comprise at least one property difference therebetween; andheating the mold contents to form an impregnated cutting structure.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an impregnated bit.

FIG. 2 is an impregnated cutting structure according to one embodimentof the present disclosure.

FIG. 3 is an impregnated cutting structure according to one embodimentof the present disclosure.

FIG. 4 is an impregnated bit according to one embodiment of the presentdisclosure.

FIG. 5 is a rib according to one embodiment of the present disclosure.

FIG. 6A-B is an impregnated bit according to one embodiment of thepresent disclosure.

DETAILED DESCRIPTION

In one aspect, embodiments disclosed herein relate to encapsulatedparticles. In other aspects, embodiments disclosed herein relate toimpregnated cutting structures or impregnated drill bits containingencapsulated particles. The use of encapsulated particles in cuttingstructures is described for example in U.S. Patent Publication No.2006/0081402 and U.S. application Ser. Nos. 11/779,083 and 11/779,104,all of which are assigned to the present assignee, and hereinincorporated by reference in their entireties.

Referring to FIG. 2, a cross-section of an embodiment of a cuttingstructure is illustrated. As shown in FIG. 3, cutting structure 200includes encapsulated particles 210 and encapsulated particles 220. Eachencapsulated particle 210, 220 is formed of an abrasive particle 212,222 coated or surrounded by encapsulating shell 214, 224 of matrixpowder material. Referring to FIG. 3, a cross-section of an embodimentof an alternative cutting structure is illustrated. As shown in FIG. 4,similar to FIG. 2, cutting structure 300 includes encapsulated particles310 and encapsulated particles 320, where each encapsulated particle310, 320 is formed of an abrasive particle 312, 322 coated or surroundedby encapsulating shell 314, 324 of matrix powder material. Additionally,a third matrix powder material 304 is infiltrated through the cuttingstructure 300.

As shown in FIGS. 2 and 3, encapsulated particles 210, 310 differ fromencapsulated particles 220, 320 in several ways; however, in aparticular embodiment, only one difference between two (or otherwisemultiple) encapsulated particles in a cutting structure need exist. Ofthe types of differences that may exist, variation in total encapsulatedparticle size; matrix material composition, shell thickness, or wearproperties; or abrasive particle type, size, strength, or retentioncoating may exist among encapsulated particles. As illustrated,encapsulated particles 210, 310 differ from encapsulated particles 220,320, for example, in their total size, in the size of abrasive particles212, 312 as compared to abrasive particles 222, 322, and the type ofencapsulating shell 214, 314 as compared to encapsulating shell 224,324. However, one of ordinary skill would appreciate that othercombinations of the component parts may be used, depending on theparticular application. Each of these component parts will be furtherdiscussed, including a description of embodiments of various impregnatedcutting structures.

Abrasive Particles

In some embodiments, abrasive particles may be selected from syntheticdiamond, natural diamond, reclaimed natural or synthetic diamond grit,silicon carbide, aluminum oxide, tool steel, boron carbide, cubic boronnitride (CBN), thermally stable polycrystalline diamond (TSP), orcombinations thereof.

The shape of the abrasive particles may also be varied as abrasiveparticles may be in the shape of spheres, cubes, irregular shapes, orother shapes. In some embodiments, abrasive particles may range in sizefrom 0.2 to 2.0 mm in length or diameter; from 0.3 to 1.5 mm in otherembodiments; from 0.4 to 1.2 mm in other embodiments; and from 0.5 to1.0 mm in yet other embodiments.

However, particle sizes are often measured in a range of mesh sizes, forexample −40+80 mesh. The term “mesh” actually refers to the size of thewire mesh used to screen the particles. For example, “40 mesh” indicatesa wire mesh screen with forty holes per linear inch, where the holes aredefined by the crisscrossing strands of wire in the mesh. The hole sizeis determined by the number of meshes per inch and the wire size. Themesh sizes referred to herein are standard U.S. mesh sizes. For example,a standard 40 mesh screen has holes such that only particles having adimension less than 420 μm can pass. Particles having a size larger than420 μm are retained on a 40 mesh screen and particles smaller than 420μm pass through the screen. Therefore, the range of sizes of theparticles is defined by the largest and smallest grade of mesh used toscreen the particles. Particles in the range of −16+40 mesh (i.e.,particles are smaller than the 16 mesh screen but larger than the 40mesh screen) will only contain particles larger than 420 μm and smallerthan 1190 μm, whereas particles in the range of −40+80 mesh will onlycontain particles larger than 180 μm and smaller than 420 μm. Thus, insome embodiments, abrasive particles may include particles not largerthan would be filtered by a screen of 10 mesh. In other embodiments,abrasive particles may range in size from −15+35 mesh. In a particularembodiment, a first encapsulated particle, i.e., encapsulated particles210, 310 as shown in FIGS. 2 and 3, may include abrasive particles,i.e., particles 212, 312, ranging in size from −20+25 mesh, while asecond capsulated particle, i.e., encapsulated particles 220, 320 asshown in FIGS. 2 and 3, may include abrasive particles, i.e., particles222, 322, ranging in size from −25+35 mesh. However, one of ordinaryskill would recognize that the particle sizes and distribution of theparticle sizes of the abrasive particles may be selected to allow for abroad, uniform, or bimodal distribution, for example, depending on aparticular application, and that size ranges outside the distributiondiscussed above may also be selected. Further, although particle sizesor particle diameters are referred to, it is understood by those skilledin the art that the particles may not necessarily be spherical in shape.

Further, as discussed above, various abrasive particles that may beselected for use in the encapsulated may vary in type (i.e., chemicalcomposition) such that the multiple types of encapsulated particles mayuse different types of abrasive particles; however, one of ordinaryskill in the art would appreciate that among these particles, there mayalso be a difference in compressive strength of the particles. Forexample, some synthetic diamond grit may have a greater compressivestrength than natural diamond grit and/or reclaimed grit. Furthermore,even within the general synthetic grit type, there may exist differentgrades of grit having differing compressive strengths, such as thosegrades of grit commercially available from Element Six Ltd. (Berkshire,England).

In addition to varying the strength of the abrasive particles, thepresence and identity of an interior, retention coating on the surfaceof the abrasive particle may also optionally be varied. Thus, in someembodiments, one type of encapsulated particle formed from abrasiveparticles having an interior, retention coating thereon may be used incombination with another type of encapsulated particle formed fromabrasive particles which do not have an interior, retention coating. Inother embodiments, different coatings may be used between theencapsulated particle type, such as for example, a weaker PVD coating onabrasive particles in a first type of encapsulated particles, and astronger CVD coating on abrasive particles in a second type ofencapsulated particles. Such interior coatings may be applied byconventional techniques such as CVD or PVD, and are in contrast to theencapsulating outer shell of matrix powder material used in embodimentsof the present disclosure. One of ordinary skill in the art wouldappreciate that the interior, thin coatings (having a thickness of onlya few micrometers as compared to the thicker encapsulating shell) may bemore helpful for high temperature protection (e.g., SiC coatings) whileothers are helpful for grit retention (e.g., TiC). In certainembodiments, the “interior” coating (TiC in the above example) may helpbond the diamond to the “outer” matrix coating. Additionally, in certainapplications the interior coating may reduce thermal damage to theparticles.

Encapsulating Shell

The encapsulating shell of matrix powder material may include a mixtureof a carbide compounds and/or a metal alloy using any technique known tothose skilled in the art. For example, encapsulating matrix material mayinclude at least one of macrocrystalline tungsten carbide particles,carburized tungsten carbide particles, cast tungsten carbide particles,and sintered tungsten carbide particles. In other embodimentsnon-tungsten carbides of vanadium, chromium, titanium, tantalum,niobium, and other carbides of the transition metal group may be used.In yet other embodiments, carbides, oxides, and nitrides of Group IVA,VA, or VIA metals may be used. A binder powder may also optionallyinclude a binder powder that may, for example, include cobalt, nickel,iron, chromium, copper, molybdenum and other transition elements andtheir alloys, and combinations thereof. Further, a non-metallic binderphase, such as polyethylene glycol (PEG) or organic wax.

Tungsten carbide is a chemical compound containing both the transitionmetal tungsten and carbon. This material is known in the art to haveextremely high hardness, high compressive strength and high wearresistance which makes it ideal for use in high stress applications. Itsextreme hardness makes it useful in the manufacture of cutting tools,abrasives and bearings, as a cheaper and more heat-resistant alternativeto diamond.

Sintered tungsten carbide, also known as cemented tungsten carbide,refers to a material formed by mixing particles of tungsten carbide,typically monotungsten carbide, and particles of cobalt or other irongroup metal, and sintering the mixture. In a typical process for makingsintered tungsten carbide, small tungsten carbide particles, e.g., 1-15micrometers, and cobalt particles are vigorously mixed with a smallamount of organic wax which serves as a temporary binder. An organicsolvent may be used to promote uniform mixing. The mixture may beprepared for sintering by either of two techniques: it may be pressedinto solid bodies often referred to as green compacts; alternatively, itmay be formed into granules or pellets such as by pressing through ascreen, or tumbling and then screened to obtain more or less uniformpellet size.

Such green compacts or pellets are then heated in a vacuum furnace tofirst evaporate the wax and then to a temperature near the melting pointof cobalt (or the like) to cause the tungsten carbide particles to bebonded together by the metallic phase. After sintering, the compacts arecrushed and screened for the desired particle size. Similarly, thesintered pellets, which tend to bond together during sintering, arecrushed to break them apart. These are also screened to obtain a desiredparticle size. The crushed sintered carbide is generally more angularthan the pellets, which tend to be rounded.

Cast tungsten carbide is another form of tungsten carbide and hasapproximately the eutectic composition between bitungsten carbide, W₂C,and monotungsten carbide, WC. Cast carbide is typically made byresistance heating tungsten in contact with carbon, and is available intwo forms: crushed cast tungsten carbide and spherical cast tungstencarbide. Processes for producing spherical cast carbide particles aredescribed in U.S. Pat. Nos. 4,723,996 and 5,089,182, which are hereinincorporated by reference. Briefly, tungsten may be heated in a graphitecrucible having a hole through which a resultant eutectic mixture of W₂Cand WC may drip. This liquid may be quenched in a bath of oil and may besubsequently comminuted or crushed to a desired particle size to formwhat is referred to as crushed cast tungsten carbide. Alternatively, amixture of tungsten and carbon is heated above its melting point into aconstantly flowing stream which is poured onto a rotating coolingsurface, typically a water-cooled casting cone, pipe, or concaveturntable. The molten stream is rapidly cooled on the rotating surfaceand forms spherical particles of eutectic tungsten carbide, which arereferred to as spherical cast tungsten carbide.

The standard eutectic mixture of WC and W₂C is typically about 4.5weight percent carbon. Cast tungsten carbide commercially used as amatrix powder typically has a hypoeutectic carbon content of about 4weight percent. In one embodiment of the present invention, the casttungsten carbide used in the mixture of tungsten carbides is comprisedof from about 3.7 to about 4.2 weight percent carbon.

Another type of tungsten carbide is macro-crystalline tungsten carbide.This material is essentially stoichiometric WC. Most of themacro-crystalline tungsten carbide is in the form of single crystals,but some bicrystals of WC may also form in larger particles. Singlecrystal monotungsten carbide is commercially available from Kennametal,Inc., Fallon, Nev.

Carburized carbide is yet another type of tungsten carbide. Carburizedtungsten carbide is a product of the solid-state diffusion of carboninto tungsten metal at high temperatures in a protective atmosphere.Sometimes it is referred to as fully carburized tungsten carbide. Suchcarburized tungsten carbide grains usually are multi-crystalline, i.e.,they are composed of WC agglomerates. The agglomerates form grains thatare larger than the individual WC crystals. These large grains make itpossible for a metal infiltrant or an infiltration binder to infiltratea powder of such large grains. On the other hand, fine grain powders,e.g., grains less than 5 μm, do not infiltrate satisfactorily. Typicalcarburized tungsten carbide contains a minimum of 99.8% by weight of WC,with total carbon content in the range of about 6.08% to about 6.18% byweight.

According to one embodiment of the present disclosure, the encapsulatingshell of a first encapsulated particle is chosen to be different fromthe encapsulating shell of a second encapsulated particle. Thisdifference(s) between the matrix powder materials of the encapsulatedparticles may include variations in chemical make-up or particle sizeranges/distribution, which may translate, for example, into a differencein wear or erosion resistance properties of the encapsulating shell.Thus, for example, different types of carbide (or other hard) particlesmay be used among the different types of encapsulated particles. One ofordinary skill in the art would appreciate that a particular variety oftungsten carbide, for example, may be selected based on hardness/wearresistance. Further, chemical make-up may also be varied by altering thepercentage s/ratios of the amount of hard particles as compared tobinder powder. Thus, by decreasing the amount of tungsten carbideparticle and increasing the amount of binder powder in an encapsulatingshell, a softer encapsulating shell may be obtained, and vice versa.

Further, with respect to particle sizes, each type of matrix material(for respective types of encapsulated particles) may be individually beselected from particle sizes that may range in various embodiments, forexample, from about 1 to 200 micrometers, from about 1 to 150micrometers, from about 10 to 100 micrometers, and from about 5 to 75micrometers in various other embodiments or may be less than 50, 10, or3 microns in yet other embodiments. In a particular embodiment, eachtype of matrix material (for respective types of encapsulated particles)may have a particle size distribution individually selected from a mono,bi- or otherwise multi-modal distribution.

Thus, referring to FIG. 2, one of ordinary skill in the art wouldrecognize that the wear properties of an encapsulating shell 214relative to an encapsulating shell 224 may be tailored by changing theirrespective chemical makeup. Depending on the anticipated final use ofthe cutting structure, encapsulating shell 214 may be softer and lesswear resistant than encapsulating shell 224. In another embodiment,encapsulating shell 214 may be substantially softer and less wearresistant than encapsulating shell 224. In such an embodiment, therelative ease of erosion of encapsulating shell 214 would allow theabrasive particles 212 to be exposed to the formation quickly, whileproviding the bit matrix with increased durability and life throughencapsulating shell 224, which does not wear or erode near as quickly.Thus, variable wear among the encapsulated particle may allow for dualoptimization of rate of penetration (ROP) and durability, which areotherwise inapposite performance characteristics. That is, for increasedROP, increased rates of diamond exposure are necessary (and thus lesswear resistance of the matrix material in which the diamonds areimpregnated); however, for durability, greater wear resistance of thematrix material is desirable so that the bit does not wear away asquickly. Further, the variable wear of the encapsulating shell may alsoallow for fluid pathways to be created in the cutting structure that mayallow for efficient cuttings removal. However, as the variable wear ison a micro-level (the material around neighboring abrasive particle inthe formed cutting structure may possess differential wear properties,as compared to a larger or macro-region), the fluids channels that formare similarly on such a micro-level.

A desirable shell thickness may vary depending on the final intended useof the cutting structure. Also, the thickness may vary depending on thesizes of abrasive grit used in forming encapsulated particle. In someembodiments, an encapsulating shell may have an average thicknessranging from 0.1 to 1.5 mm. In other embodiments, a shell may have anaverage thickness ranging from 0.1 to 1.3 mm; from 0.15 to 1.1 mm inother embodiments; and from 0.2 to 1.0 mm in yet other embodiments. Inmost embodiments, a shell may have an average thickness ranging from 750micrometers to 1000 micrometers.

Further, while the encapsulated particles are shown in FIGS. 2 and 3 ashaving shells of approximately the same thickness, the present inventionis not so limited. The thickness and chemical composition of theencapsulating shells may be tailored to achieve a desirable wear rate.Thus, abrasive grits may be of different sizes or of different kinds,and the encapsulating shells may be of various thicknesses and comprisematrices, which may wear at different rates thereby exposing the gritsat different rates. The composition and thickness of this second matrixmay also affect the rate at which the encapsulated grit is exposed. Forexample, it may take a longer time to expose the abrasive grit in anencapsulated particle with a larger shell thickness than a grit with asmaller shell thickness of same chemical composition.

Encapsulated Particles

Encapsulated particles may be formed by encapsulating or coatingabrasive particles with matrix powder material using encapsulationtechniques known to one skilled in the art. In a particular embodiment,at least two types of encapsulated particles are used to form a variableimpregnated cutting structure. In embodiments where two types ofencapsulated particles are used, the ratio of those particles may rangefrom 20:80 to 80:20 in one embodiment, and 30:70-70:30 in anotherembodiment. However, one of ordinary skill in the art would appreciatethat other number of types of encapsulated particles may find use in thecutting structures of the present disclosure. For example, where threeencapsulated particle types are desired, a first particle type mayrepresent 5 to 30 percent, a second particle type representing 10-40percent, and a third particle type representing 30-85 percent of thetotal amount of encapsulating particles. However, one of ordinary skillin the art would appreciate the particular combination of encapsulatedparticle types and amounts may be varied depending on the particularapplication.

In some embodiments, each type of encapsulated particle may have anindividually selected average diameter (or equivalent diameter) rangingfrom 0.3 to 3.5 mm. In other embodiments, encapsulated particles mayhave an average diameter ranging from 0.4 to 3.0 mm; from 0.5 to 2.5 mmin other embodiments; and from 0.7 to 2.0 mm in yet other embodiments.In other embodiments, encapsulated particles 38 may include particlesnot larger than would be filtered by a screen of 5 mesh. In otherembodiments, encapsulated particles may range in size from −10+25 mesh.While the encapsulated particles are primarily shown as spheres, one ofordinary skill in the art would appreciate that the present disclosureis not so limited.

In various embodiments, encapsulated particles may be obtained fromcommercial sources, or synthesized using encapsulation techniques knownto those of ordinary skill in the art.

Infiltrating Matrix Material

For embodiments where an infiltrating matrix material is used, theinfiltrating matrix material may include hard particles and a binderphase. Such exemplary hard particles include tungsten (W) or aderivative such as tungsten carbide (WC), sintered tungstencarbide/cobalt (WC—Co) (spherical or crushed), cast tungsten carbide(particulate or crushed), macro-crystalline tungsten carbide, carburizedtungsten carbide, other carbides, or combinations of these materialswith an optional binder. In other embodiments, the infiltrating matrixmaterial may be formed from hard particle materials such as carbides ornitrides of tungsten, vanadium, boron, titanium, or combinationsthereof. Typically, a binder phase may be formed from a powder componentand/or an infiltrating component. In some embodiments of the presentinvention, hard particles may be used in combination with a powderbinder such as cobalt, nickel, iron, chromium, copper, molybdenum andtheir alloys, and combinations thereof. In various other embodiments,the first matrix material 44 may include a Cu—Mn—Ni alloy,Ni—Cr—Si—B—Al—C alloy, Ni—Al alloy, and/or Cu—P alloy. In otherembodiments, the infiltrating matrix material may include carbides inamounts ranging from 50 to 70% by weight in addition to at least onebinder in amount ranging from 30 to 50% by weight thereof to facilitatebonding of matrix material and impregnated materials. In one embodiment,the resulting infiltrating matrix material may be chosen to be verytough, yet maintain good cutting properties. Additionally, tungstencarbide, in particular a fine-grained tungsten carbide, may present anoptimum matrix for controlled wear and cuttings removal.

In various embodiments, the infiltrating matrix may include hardparticles ranging in size from about 1 to 200 micrometers, or about 5 to150 micrometers, or about 10 to 100 micrometers. One of ordinary skillin the art would recognize that the particular combination of hardparticle material and particle size used in the matrix material maydepend, for example, on whether the particles disclosed herein are beingused in a insert or a rib of a bit body so that desired properties suchas wear resistance and ability to be infiltrated may be optimized.

One of ordinary skill in the art would recognize that the particularcombination of carbides and binders used in the infiltrating matrixmaterial may be tailored depending on the anticipated final use of thecutting structure. For example, the combination used may be customizedfor desired properties such as wear resistance and ability to beinfiltrated. The infiltrating matrix material may be chosen to havesufficient hardness so that the impregnated materials, namely theencapsulated particles, exposed at the cutting face are not pushed intothe matrix material under the very high pressures commonly encounteredin drilling. In addition, the infiltrating matrix material may beselected to withstand continuous mechanical action such as rubbing,scraping, or erosion that typically occurs during drilling so that theimpregnated materials are not prematurely released.

Manufacture of Cutting Structures Using Encapsulated Particles

In one embodiment, uniformly coated encapsulated particles aremanufactured prior to the formation of the impregnated bit. An exemplarymethod for achieving “uniform coatings” is to mix the abrasiveparticles, and a matrix material in a commercial mixing machine such asa Turbula Mixer or similar machine used for blending diamonds withmatrix. The resultant mix may then be processed through a “granulator”in which the mix is extruded into short “sausage” shapes which are thenrolled into balls and dried. The granules that are so formed must beseparated using a series of mesh screens in order to obtain the desiredyield of uniformly coated particles. At the end of this process, anumber of particles of approximately the same size and shape can becollected, and optionally pre-sintered. Another exemplary method forachieving a uniform matrix coating on the abrasive grits is to use amachine called a Fuji Paudal pelletizing machine. The uniformly coatedparticles may then be transferred into a mold cavity and formed into aninsert or other cutting structure, i.e., rib of a drill bit. One suchprocess is described in U.S. Patent Application Publication No.2006/0081402, which is herein incorporated by reference in its entirety.

One of ordinary skill in the art would appreciate that the encapsulatedparticles disclosed herein may be used to form inserts, cuttingstructures or bit bodies using any suitable method known in the art.Heating of the material can be by furnace or by electric inductionheating, such that the heating and cooling rates are rapid andcontrolled in order to prevent damage to the diamonds. The inserts maybe heated by resistance heating in a graphite mold, while bit bodies maybe formed by infiltration of a mold. The dimensions and shapes of theinserts and of their positioning on the bit can be varied, depending onthe nature of the formation to be drilled.

Infiltration processes that may be used to form an infiltrated bit bodyof the present disclosure may begin with the fabrication of a mold,having the desired body shape and component configuration. Pellets ofuniformly coated encapsulated particles may be loaded into the mold inthe desired location, i.e., ribs, and, a matrix material, and optionallya metal binder powder, may be loaded on top of the encapsulatedparticles. The mass of particles may be infiltrated with a molteninfiltration binder and cooled to form a bit body. In a particularembodiment, during infiltration at least a portion of the loaded matrixmaterial may be carried down with the molten infiltrant to fill the gapsbetween the encapsulated particles. Depending on the size of theencapsulated particles, as well as additional properties, a sizedistribution of the additional matrix material may be likewise selectedsuch that the additional matrix material possess a sufficient amount of“fine” particles that may be carried down between the encapsulatedparticles to fill the gaps therebetween.

It will further be understood that the concentration of diamond orabrasive particles in a consolidated insert, for example, can differfrom the concentration of diamond or abrasive particles in the bit body.Diamond concentration may be obtained, for example by varying shellthickness and the matrix loading of the first matrix material. Accordingto one embodiment, the concentrations of diamond in the inserts and inthe bit body are in the range of 50 to 120 (100=4.4 carat/cm³). Otherembodiments may have a diamond concentration greater than 110, while yetother embodiments may have a diamond concentration less than 85. Adiamond concentration of 120 is equivalent to 30 percent by volume ofdiamond. Those having ordinary skill in the art will recognize thatother concentrations of diamonds may also be used depending onparticular applications. Further, in some embodiments, the various typesof encapsulated particles may have a varied concentration, such as aconcentration of at least 110 for one type of particle, and aconcentration of at most 100 for another type of particles. However, oneof ordinary skill in the art would appreciate that other combinationsmay be used.

Further, while reference has been made to a hot-pressing process above,embodiments disclosed herein may use a high-temperature, high-pressurepress (HTHP) process. Alternatively, a two-stage manufacturingtechnique, using both the hot-pressing and the HTHP, may be used topromote the development of high concentration (>120 conc.) whileachieving maximum bond or matrix density. The HTHP press can improve theperformance of the final structure by enabling the use of higher diamondvolume percent (including bi-modal or multi-modal diamond mixtures)because ultrahigh pressures can consolidate the bond material to nearfull density (with or without the need for low-melting alloys to aidsintering).

The HTHP process has been described in U.S. Pat. No. 5,676,496 and U.S.Pat. No. 5,598,621. Another suitable method for hot-compactingpre-pressed diamond/metal powder mixtures is hot isostatic pressing,which is known in the art. See Peter E. Price and Steven P. Kohler, “HotIsostatic Pressing of Metal Powders”, Metals Handbook, Vol. 7, pp.419-443 (9th ed. 1984).

Further, the processing times during sintering or hot-pressing, such asheating and cooling times, may be selected to be sufficiently short, aswell as the maximum temperature of the thermal cycle may be selected tobe sufficiently low, so that the impregnated materials are not thermallydamaged during these processes.

In some embodiments, the multiple types of encapsulated particles on ribmay include particles of varying size, varying composition, orcombinations thereof. In other embodiments, the multiple encapsulatedparticles may include shells of varying thickness, varying composition,or combinations thereof. In yet other embodiments, the multipleencapsulated particles may include abrasive particles of varying size,varying composition, varying size distribution, and combinationsthereof. In yet other embodiments, the drill bit or a rib on a drill bitmay additionally include (be impregnated with) standard grit.

In various embodiments, the encapsulated particles disclosed herein mayhave localized placement in a drill bit. For example, encapsulatedparticles may be placed at the top of the bit being the first section ofthe bit to drill or solely imbedded deeper within the bit for drillingof the latter sections encountered during a bit run. Additionally, oneof skill in the art would recognize that it may be advantageous to placethe encapsulated particles at other strategic positions, such as, forexample, in the gage area, and leading, or trailing sides of arib/blade.

Further, as discussed above, the encapsulated particles may be used in aconsolidated or hot pressed insert, such as the type described in U.S.Pat. No. 6,394,202, which is assigned to the present assignee and hereinincorporated by reference in its entirety. As shown in FIG. 4, suchinserts may be inserted into a drill bit. Bit 422 includes a shank 424and a crown 426. Crown 426 has a cutting face 429 and outer side surface430. According to one embodiment, crown 426 is formed by infiltrating amass of tungsten-carbide powder impregnated with synthetic or naturaldiamond, or alternatively by infiltrating encapsulated particles asdescribed herein.

Crown 426 may include various surface features, such as ribs 427, whichmay optionally be formed with spacers in the mold during themanufacturing process so that the infiltrated, diamond-impregnated crownincludes a plurality of holes or sockets 434 that are sized and shapedto receive a corresponding plurality of diamond-impregnated inserts 436.Once crown 426 is formed, inserts 436 formed from the encapsulatedparticles of the present disclosure may be mounted in the sockets 434and affixed by any suitable method, such as brazing, adhesive,mechanical means such as interference fit, or the like. As shown in FIG.4, the sockets 434 may each be substantially perpendicular to thesurface of the crown 426 so that once inserted into sockets 434, insertsare substantially perpendicular to the surface of crown 426 (and may beflush with or extend beyond surface of crown 426). Alternatively, holes434 can be inclined with respect to the surface of the crown 426. Inthis embodiment, the sockets are inclined such that inserts 436 areoriented substantially in the direction of rotation of the bit, so as toenhance cutting.

Alternatively, inserts may be stacked within a rib, as shown in FIG. 5.Specifically, a rib 527 may include a plurality of inserts 536 (formedfrom encapsulated particles as disclosed herein) stacked within the rib,along its length, in a side by side fashion. Thus, the particularorientation of the diamond impregnated inserts of the present disclosurewithin a bit does not have any limitation on the scope of the presentdisclosure.

Further, it is also within the scope of the present disclosure that abit is formed without impregnated inserts, but with the encapsulatedparticle loaded into bit mold cavity and infiltrated, as describedabove. FIGS. 6A-B illustrate partial views of an alternative embodimentof an impregnated bit, generally indicated by arrow 610. In FIGS. 6A-B,approximately one half of the circular face 611 is shown, with the otherhalf being approximately the mirror image of the part that is shown. Thebit body 613 is cylindrical in form, with the upper end thereof (notshown) forming a threaded pin which is adapted to be connected to thelower end of a drill string. The lower end of the bit body 613 forms theend face 611. The end face 611 has a plurality of elevated ribs 165formed thereon, with channels 617 formed between the ribs 615.

The bit body 613 is preferably made of a steel core 612 having an outershell 614 comprised of matrix material. The ribs 615 include diamonds(not illustrated) embedded within a matrix material, where the rib wasformed from the encapsulated particles of the present disclosure. Thediamond particles then function to wear away the bore hole formation asthe bit rotates. The channels 617 function to allow drilling fluid topass through a central plenum (not shown) from the interior of the bitbody 613 and run along the channels 617 to cool the ribs 615 and tocarry the formation cuttings up the annulus formed between the bit andthe bore hole. Thus, in forming the bit, encapsulated particles and amatrix material are loaded into a mold cavity and heated, such as byinfiltration or sintering, to form the resulting impregnated bit.Further, as the use of encapsulated particles may allow for a uniquetailoring of the bit composition, taller ribs (for a given rib width)may be obtained, without high risk of failure by rib breakage. Theinclusion of tall ribs may be determined by the ratio of the rib height(indicated as 640 on FIGS. 6A-B) to rib width (indicated as 650 on FIGS.6A-B). Conventional bit designs generally require a ratio of rib heightto width of no more than 1.5. However, by using at least one type ofencapsulated particles (or at least two in other embodiments) of thepresent disclosure, bit having a ratio of rib height to rib width (alongany portion of the rib) of greater than 1.75 may be obtained using aminimum diamond concentration of 100. In other embodiments, a ratio ofrib height to width of greater than 2.0 may be obtained using a minimumdiamond concentration of 75. In yet other embodiments, a ratio of ribheight to width of greater than 2.25 may be obtained using a minimumdiamond concentration of 50. Such ratios translate, for example, to arib height of greater than 30 mm on an 8⅜ inch bit. In variousembodiments, the specified ratio of bit height to width refers to aportion of the rib along the bit center (nose) or bit top (nose). Mostof the catastrophic rib breakage typically occurs near these areas, thebit center (cone) and bit top (nose), because ribs typically have lesswidth at these locations as compared to the gage area due to spacerestrictions and load concentrations when used in drilling.

Use of the encapsulated particles disclosed herein may provide increasedTRS bending strength of at least 15 percent in some embodiment, at least20 or 25 percent in other embodiments, and at least 30 percent in yetother embodiments. Further, one skilled in the art would appreciate thatthe recited ratios may be obtained by either reducing the rib width (tofit more ribs and thus more surface area for wear resistance) and/or byincreasing blade height (to increase bit life before tripping), both ofwhich may use the encapsulated particles disclosed herein.

Referring again to FIGS. 5-6B, impregnated bits may include a pluralityof gage protection elements disposed on the ribs and/or the bit body. Insome embodiments, the gage protection elements may be modified toinclude evenly distributed diamonds. By positioning evenly distributeddiamond particles at and/or beneath the surface of the ribs, theimpregnated bits are believed to exhibit increased durability and areless likely to exhibit premature wear than typical prior art impregnatedbits.

Embodiments disclosed herein, therefore, may find use in any applicationin which impregnated cutting structures may be used. Specifically,embodiments may be used to create diamond impregnated inserts, diamondimpregnated bit bodies, diamond impregnated wear pads, or any otherdiamond impregnated material known to those of ordinary skill in theart. Embodiments may also find use as inserts or wear pads for 3-cone,2-cone, and 1-cone (1-cone with a bearing & seal) drill bits. Further,while reference has been made to spherical particles, it will beunderstood by those having ordinary skill in the art that otherparticles and/or techniques may be used in order to achieve the desiredresult, namely more even distribution of diamond particles. For example,it is expressly within the scope of the present invention thatelliptically coated particles may be used.

EXEMPLARY EMBODIMENTS

An impregnated cutting structure formed in accordance with the presentdisclosure may include two types of encapsulated particles. The firsttype of particle may constitute 35% of the total volume of particles. Itmay include synthetic grit having a mesh size of −25+35, such asSDB1100, which is a strong grit commercially available from Element SixLtd, coated with a TiC coating applied by chemical vapor deposition(CVD). A hard matrix comprising 70% WC (<10 micrometer standard WC type)and a binder mixture of Co and Cu may be used to encapsulate theparticles sufficient to form a 110 diamond concentration. The secondtype of particle may constitute 65% of the total volume of particles. Itmay include synthetic grit having a mesh size of −20+25, such as MBS950,which is a medium strength grit commercially available from DiamondInnovations, Inc., coated with a TiC or SiC coating applied by CVD. Asoft matrix comprising 30% WC (<10 micrometer standard WC type) and abinder mixture of Co and Cu may be used to encapsulate the particlessufficient to form a diamond concentration of 80. Such embodiment may beused to form hot pressed inserts, as described above, or may be used inconjunction with an infiltrating matrix material to form an impregnatedbit body.

A second impregnated cutting structure formed in accordance with thepresent disclosure may also include two types of encapsulated particles.The first type of particle may constitute 35% of the total volume ofparticles. It may include synthetic grit having a mesh size of −25+35,such as MBS960, which is a high strength grit commercially availablefrom Diamond Innovations, Inc., coated with a strong SiC coating appliedby CVD. A soft matrix comprising 30% WC (<10 micrometer standard WCtype) and a binder mixture of Co and Cu may be used to encapsulate theparticles sufficient to form a 110 diamond concentration. The secondtype of particle may constitute 65% of the total volume of particles. Itmay include synthetic grit having a mesh size of −20+25, such as MBS960,which is commercially available from Diamond Innovations, Inc., coatedwith a SiC coating applied by CVD. A soft matrix comprising 30% WC (<10micrometer standard WC type) and a binder mixture of Co and Cu may beused to encapsulate the particles sufficient to form a diamondconcentration of 100. Such embodiment may be used in particular whenhigh ROP is desired (due to the soft/high toughness nature of theencapsulating material).

A third impregnated cutting structure formed in accordance with thepresent disclosure may include three types of encapsulated particles.The first type of particle may constitute 55% of the total volume ofparticles. It may include synthetic grit having a mesh size of −18+20,such as NDG120, which is a strong grit commercially available fromElement Six Ltd, coated with a TiC coating applied by CVD. A soft matrixcomprising 30% WC (<10 micrometer standard WC type) and a binder mixtureof Co and Cu may be used to encapsulate the particles sufficient to forma 120 diamond concentration. The second type of particle may constitute30% of the total volume of particles. It may include synthetic grithaving a mesh size of −25+35, such as SBD1100, which is a strong gritcommercially available from Element Six Ltd, coated with a TiC coatingapplied by CVD. A medium hardness matrix comprising 50% WC (<10micrometer standard WC type) and a binder mixture of Co and Cu may beused to encapsulate the particles sufficient to form a diamondconcentration of 120. A third type of particle may constitute 15% of thetotal volume of particles. It may include synthetic grit having a meshsize of −35+40, such as MBS950, which is a medium strength gritcommercially available from Diamond Innovations, Inc., coated with a SiCcoating applied by CVD. A hard matrix comprising 70% WC (<100 micrometermixture of standard WC and cast WC/W₂C types) and a binder mixture of Coand Cu may be used to encapsulate the particles sufficient to form adiamond concentration of 120. Such embodiment may be used to form hotpressed inserts, as described above, or may be used in conjunction withan infiltrating matrix material to form an impregnated bit body.

Advantageously, embodiments of the present disclosure may provide for atleast one of the following. As discussed above, embodiments disclosedherein may provide more controllable wear properties, improved diamondretention, and increased diamond concentration (without diamondcluttering) for a given volume. Embodiments disclosed herein may alsoprovide for the controlled exposure of fresh grit for increased ROP, asremoval of the grit to expose fresh grit may be controlled by thehardness of the shell and the relative wear properties of the variousmatrix materials used, and may be tailored for the hardness of the earthformation.

Additionally, in the embodiments disclosed herein, the variouscombinations of encapsulated particle components that may be used mayprovide improved cutting structures to drill through formations ofspecific hardnesses and/or may make a bit particularly suitable fordrilling through a variety of formations, including mixed formations,due to the adaptive nature of the bit. Further, a first encapsulatingmatrix material may be selected for its toughness, which may reduceblade breakage and allow the blade height to increase, which wouldincrease the drilling life of the blade. Improvements in properties,such as bending strength, may be obtained by using encapsulatedparticles, which may allow for an increase in rib height to widthratios. Such ratio may be realized by either reducing rib width (toincrease the number of ribs and thus surface area for wear resistance)and/or increasing the rib height (to increase bit life before tripping).Further, by blending at least two distinct pellets to form ribs havingincreased height to width ratios, it may be possible to effectivelydrill through mixed formation types. For example, a first pellet typemay be selected to provide optimum drilling through abrasive sandstones,while a second pellet type may be selected to provide optimum drillingthrough shale, limestone, or chert (hard nodules). Typically, duringformation changes, an operator is typically forced to pull animpregnated bit (despite having remaining bit life) due to reduced ROPto drill with another bit type, such as a roller cone bit, which haveless life to bearing and seal failure. Thus, the combination ofincreased bit height and the unique encapsulated particles, may providefor increase bit life despite formation changes.

However, certain quantities of abrasive particles may be more readilyexposed by the softer encapsulating material, which also increases ROP.Further, a second matrix material may be selected to be more wearresistant than the first matrix material in order to expose theconcentrated grit at a slower rate. This may result in a robust cuttinginstrument wherein the grit is exposed in a controlled fashion.

Further, the disparity in wear properties between multiple matrices mayallow for tailoring of the some of the properties of the cuttingstructure such as grit concentration, wear rate, controlled exposure ofencapsulated grit to the formation, cuttings removal and robustness. Ifa high grit concentration is required for drilling a particularly hardformation, the shell thickness may be small. This may advantageouslyallow more encapsulated grit to be packed into the same cuttingstructure.

If more efficient cuttings removal is required, the cutting instrumentmay have a first matrix that is selected to be more wear resistant thanthe second matrix material. The second matrix may preferentiallypartially wear away creating fluid pathways within the cuttinginstrument, while exposing abrasive particles. This may result in acutting instrument with superior cuttings removal properties.

Further, conventional bits rely on grit hot pressed inserts for a largeportion of the wear; however, such segments are typically restricted toapproximately thirty to forty percent of the rib volume due to designlimitations. Because the cutting structures of the present disclosuremay provide for improved rate of penetration by virtue of improved wearpatterns, a bit that typically relies on grit hot pressed inserts forwear may instead be provided with ribs infiltrated with the encapsulatedparticles as disclosed herein. Such bits may possess improved wearacross a larger volume of rib, as compared to conventional bits havinggrit hot pressed inserts.

Thus, embodiments disclosed herein may allow for an effective diameterof the encapsulated materials without such drastic increases in cost.Furthermore, some embodiments may include a hard particle, such astungsten or silicon carbide, which has even lower costs as compared todiamond or other super abrasives. Therefore, cost savings may beachieved while maintaining or even improving rate of penetration (ROP),thus lowering the drilling cost per foot.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. An impregnated cutting structure comprising: a plurality of firstencapsulated particles, each first encapsulated particle comprising afirst abrasive particle encapsulated by a first matrix material shell;and a plurality of second encapsulated particles, the secondencapsulated particles comprising a second abrasive particleencapsulated by a second matrix material shell, wherein the firstencapsulated particles and the second encapsulated particles have atleast one property difference.
 2. The cutting structure of claim 1,wherein the first matrix material and second matrix material comprisedifferent wear rates.
 3. The cutting structure of claim 2, wherein thefirst matrix material is softer than the second matrix material.
 4. Thecutting structure of claim 1, wherein the first encapsulated particle iscoarser than the second encapsulated particle.
 5. The cutting structureof claim 1, wherein the first abrasive particle is coarser than thesecond abrasive particle.
 6. The cutting structure of claim 1, whereinthe first abrasive particle has less compressive strength than thesecond abrasive particle.
 7. The cutting structure of claim 1, whereinthe encapsulating first matrix material shell is thicker than theencapsulating second matrix material shell.
 8. The cutting structure ofclaim 1, wherein at least one of the first abrasive particle and secondabrasive particle have a retention coating deposited thereon.
 9. Thecutting structure of claim 8, wherein the first abrasive particle has aweaker retention coating deposited thereon as compared to the secondabrasive particle.
 10. The cutting structure of claim 1, where the firstand the second matrix materials individually comprise at least one ofsintered tungsten carbide, cast tungsten carbide, and carbides oftungsten, vanadium, chromium, titanium, tantalum, and niobium.
 11. Thecutting structure of claim 1, where the first and the second matrixmaterials individually comprise at least one of copper, cobalt, nickel,iron, and alloys thereof.
 12. The cutting structure of claim 1, whereinthe first and second abrasive particles individually comprise at leastone of synthetic diamond, natural diamond, TSP, and CBN.
 13. The cuttingstructure of claim 1, further comprising: a plurality of thirdencapsulated particles, each third encapsulated particle comprises athird abrasive particle encapsulated by a third matrix material shell.14. A drill bit, comprising: a bit body; and a plurality of ribs formedin the bit body, wherein at least one rib comprises: a plurality offirst encapsulated particles, each first encapsulated particlecomprising a first abrasive particle encapsulated by a first matrixmaterial shell; a plurality of second encapsulated particles, eachsecond encapsulated particle comprising a second abrasive particleencapsulated by a second matrix material shell, wherein the firstencapsulated particles and the second encapsulated particles comprise atleast one property difference therebetween.
 15. The drill bit of claim14, wherein the first matrix material and second matrix materialcomprise different wear properties.
 16. The drill bit of claim 15,further comprising: a third matrix material infiltrated between thefirst and second encapsulated materials.
 17. The drill bit of claim 14,wherein a consolidated insert comprising the first and secondencapsulated particles are brazed or cast into the rib.
 18. A drill bit,comprising: a bit body; and a plurality of ribs formed in the bit body,wherein a portion of at least one rib has a height to width ratio ofgreater than about 1.75 with a minimum diamond concentration of 100 andcomprises: a plurality of first encapsulated particles, each firstencapsulated particle comprising a first abrasive particle encapsulatedby a first matrix material shell.
 19. The drill bit of claim 18, whereinthe at least one rib further comprises: a plurality of secondencapsulated particles, each second encapsulated particle comprising asecond abrasive particle encapsulated by a second matrix material shell,wherein the first encapsulated particles and the second encapsulatedparticles comprise at least one property difference therebetween. 20.The drill bit of claim 19, wherein the first matrix material and secondmatrix material comprise different wear properties.
 21. The drill bit ofclaim 19, wherein the at least one rib further comprises: a third matrixmaterial infiltrated between the first and second encapsulatedmaterials.
 22. A method of forming an impregnated cutting structurecomprising: loading a plurality of first encapsulated particles and aplurality of second encapsulated particles into a mold cavity, eachfirst encapsulated particle comprising a first abrasive particleencapsulated by a first matrix material shell and each secondencapsulated particle comprising a second abrasive particle encapsulatedby a second matrix material shell, wherein the first encapsulatedparticles and the second encapsulated particles comprise at least oneproperty difference therebetween; and heating the mold contents to forman impregnated cutting structure.
 23. The method of claim 22, furthercomprising: applying pressure to the first and second encapsulatedparticles within the mold.
 24. The method of claim 22, furthercomprising: loading a third matrix material into the mold with the firstand second encapsulated particles; and infiltrating the mold contentswith a infiltrating binder.
 25. The method of claim 22, wherein the atleast one property difference comprises at least one of different wearrates between the matrix materials, different toughness between thematrix materials, different size of the encapsulated particles,different size of the abrasive particles, different compressivestrengths between the abrasive particles, different shell thicknesses,and presence or type of retention coatings.